[dev.boringcrypto] all: merge master into dev.boringcrypto

Change-Id: Ic5f71c04f08c03319c043f35be501875adb0a3b0

1 files changed, 18 insertions, 3 deletions

diff --git a/src/bufio/bufio.go b/src/bufio/bufio.go
index 7483946fc0a..bcc273c78b2 100644
--- a/src/bufio/bufio.go
+++ b/src/bufio/bufio.go
@@ -731,13 +731,28 @@ func (b *Writer) WriteRune(r rune) (size int, err error) {
 // If the count is less than len(s), it also returns an error explaining
 // why the write is short.
 func (b *Writer) WriteString(s string) (int, error) {
+	var sw io.StringWriter
+	tryStringWriter := true
+
 	nn := 0
 	for len(s) > b.Available() && b.err == nil {
-		n := copy(b.buf[b.n:], s)
-		b.n += n
+		var n int
+		if b.Buffered() == 0 && sw == nil && tryStringWriter {
+			// Check at most once whether b.wr is a StringWriter.
+			sw, tryStringWriter = b.wr.(io.StringWriter)
+		}
+		if b.Buffered() == 0 && tryStringWriter {
+			// Large write, empty buffer, and the underlying writer supports
+			// WriteString: forward the write to the underlying StringWriter.
+			// This avoids an extra copy.
+			n, b.err = sw.WriteString(s)
+		} else {
+			n = copy(b.buf[b.n:], s)
+			b.n += n
+			b.Flush()
+		}
 		nn += n
 		s = s[n:]
-		b.Flush()
 	}
 	if b.err != nil {
 		return nn, b.err